1. Field of the invention
This invention relates to an ink-jet recording head having a plurality of thermoelectric transducers in one flow path (nozzle), and an ink-jet recording apparatus employing such a head.
2. Related Background Art
Ink-jet recording apparatus are mostly used as printing apparatus in printers, facsimile machines, word processors, copying machines and so forth. In particular, ink-jet recording apparatus of a system in which heat energy is used as an energy utilized for the discharge of ink and bubbles are generated by the heat energy to discharge the ink are recently coming into wide use.
As a different use of the ink-jet recording apparatus of this system, ink-jet textile printing apparatus that can print patterns, designs or composite images are also recently becoming available. Ink-jet recording heads used in the ink-jet recording apparatus described above make use of thermoelectric transducers (hereinafter also "heaters") as means for generating the heat energy, and, in many instances, employ the construction wherein one heater is provided for one discharge opening. In contrast thereto, heads having a plurality of heads for one discharge opening from the viewpoint of the following are also known in the art.
That is, what is known in the first place is a head comprising a plurality of heaters which are driven alternately or one by one so that the ink-jet recording head can have a long lifetime. Secondly, it is a head making use of a plurality of heaters so that the quantity of ink discharge (discharge quantity) can be changed over a broader range, where the discharge quantity is changed by selecting the heaters to be driven or the number thereof.
Of the heads described above, the latter has, as a more specific construction, a plurality of heaters arranged in the direction of ink discharge in an ink flow path communicating with the discharge opening of the ink-jet recording head, where the heater to be driven (heat-generated) or the number of the heater are selected to make different the distance between the discharge opening and the heater(s) to be driven to thereby change the discharge quantity. Such structure is known in the art.
As a different construction, a head is also known in which, as disclosed, e.g., in Japanese Patent Application Laid-open No. 55-132259, a plurality of heaters having surface areas different from one another are arranged in its ink flow path, and similarly the heater to be driven or the number thereof is changed to make the discharge quantity changeable.
There, however, are some problems in order to accomplish such ink-jet recording apparatus made changeable in the discharge quantity.
One problem is that, when ink droplets with a small discharge quantity are discharged, the ink is caused to bubble by a heater having a small discharge power, i.e., a small heater area, and hence the ink is discharged not only in a small discharge quantity but also at a low discharge velocity. This can be a great problem especially with regard to what is called preliminary discharge performed as a part of operation for discharge restoration.
The preliminary discharge is performed usually when the apparatus is in a certain specified state, and the ink not participating in the recording is discharged from the ink-jet recording head, whereby any ink having thickened in the ink-jet recording head is removed so that the ink discharge can be kept in a good state. Usually, such preliminary discharge is performed at given time intervals immediately after the power source of the apparatus is turned on or in the course of recording.
However, in the instance where the ink can be discharged in various discharge quantities by a plurality of heaters as stated above, the preliminary discharge must be performed at short intervals when the recording is performed in the discharge quantity set small. More specifically, since the ink droplets with a small discharge quantity have a small power, there is a possibility that the thick ink can not be stably discharged, depending on how the ink has thickened as a result of the evaporation of water at the discharge opening. Hence, it becomes necessary to frequently perform the preliminary discharge. In particular, the preliminary discharge performed at given time intervals in the course of recording must be done at shorter intervals, bringing about the problem of a decrease in through-put of the recording.
As another problem, discharge of ink droplets at a low velocity may cause more or less a change of the direction of discharge from the discharge opening to cause a lowering of the performance of ink-shot against a recording medium, tending to adversely affect the image quality.
As still another problem, it has been found that the discharge velocity is substantially proportional to the discharge quantity, thus a difference in discharge velocity is great when the discharge quantity is large or small. When images are formed using such an ink-jet recording head, the large dots and small dots formed by the ink droplets discharged in a large discharge quantity and a small discharge quantity are shot at different positions because of the difference in discharge velocity, resulting in an increase in the possibility of faulty images.
In addition to the above problems, the conventional ink-jet recording heads have the following problems.
When images are recorded in a small discharge quantity, their resolution increases and hence the amount of image data increases, resulting in an increase in recording dots. Accordingly, the ink must be discharged at a higher repetition frequency in order to maintain or increase recording speed. For some types of ink used, it is very difficult to do so.
It is also important that the large-quantity discharge is made at a high repetition frequency. This is because the recording apparatus results in spec-down if the recording in conventional discharge quantity is performed at a low speed, even though the recording in small discharge quantity can be performed at a high speed.
From a different point of view, it is also important for ink-jet recording heads to be of types common to former types. This is due to, e.g., the fact that ink-jet recording heads having a stated discharge quantity are used as exchange types. If, for the purpose of improving the function of recording apparatus, an ink-jet recording head is so set up that not only ink droplets in a large discharge quantity but also those in a small discharge quantity are discharged using the same head and the head is not of the type common to former types, a former type ink-jet recording head and a new type ink-jet recording head for discharging ink droplets of large discharge quantity and small discharge quantity must be produced in parallel, bringing about problems of an increase in production lines and an increase in production cost.